The effects of mineralocorticoids, arginine vasopressin, and bradykinin on isolated perfused cortical collecting ducts from rats are being investigated. Chronic deoxycorticosterone treatment of the rats increases sodium absorption and potassium secretion by the collecting ducts in vitro. Addition of arginine vasopressin to the bath in vitro increases the rates of sodium, chloride, bicarbonate, and fluid absorption and the rate of potassium secretion. Addition of bradykinin to the bath in vitro inhibited the sodium and chloride absorption, but did not significantly affect potassium or bicarbonate transport, or the transepithelial potential difference. These results are consistent with the presence of an electroneutral NaCl transport process in rat cortical collecting ducts that is inhibited by bradykinin.